1. Field of the Invention
The present invention relates to a negative pressure control apparatus for a brake booster, and particularly to a negative pressure control apparatus for controlling negative pressure of a brake booster which assists a brake operation by using intake manifold negative pressure of a direct-injection engine as a power source.
2. Description of the Related Art
Conventionally, as disclosed in Japanese Laid-Open Patent Application No. 5-208663, a braking apparatus having a brake booster is known. The brake booster is a mechanism which assists a brake operation by using an intake manifold negative pressure as a power source so that a larger braking force can be generated.
Generally, in a case of a regular engine which controls an opening of a throttle valve in accordance with an acceleration operation, when a driver intends to decelerate a vehicle, that is, when an accelerator pedal is not depressed, the throttle valve is closed so that a relatively large intake manifold negative pressure is generated. Thus, when the driver performs a brake operation, the brake booster can be positively operated.
The above-mentioned conventional braking apparatus has a function of performing an automatic brake control for generating a braking force irrespective of whether or not a brake operation is performed when an obstacle is detected ahead of the vehicle. In the above-mentioned conventional braking apparatus, the brake booster is constructed so that it can generate a required braking force when no brake operation is performed. However, if the automatic brake control is performed when the driver intends to accelerate the vehicle, that is, when the throttle valve is opened, the intake manifold negative pressure is decreased. In this case, the negative pressure in the brake booster may become insufficient and the brake booster in the automatic brake control may not generate the required braking force. In order to avoid such a problem, the above conventional braking apparatus generates an intake manifold negative pressure which is sufficient for performing the automatic brake control by forcibly closing the throttle valve when an establishment of a condition for starting the automatic brake control is predicted.
Conventionally, a direct-injection engine is known which has a fuel injector disposed inside a combustion chamber and directly injects fuel in the combustion chamber. According to the direct-injection engine, fuel economy can be improved by fully opening a throttle valve so that a pumping loss of the engine is reduced when, for example, the engine is operating under a low load. Thus, in the direct-injection engine, the intake manifold negative pressure may be decreased by the throttle valve being fully opened when an acceleration operation is not being performed. Consequently, in a vehicle having the direct-injection engine, the negative pressure in the brake booster may become insufficient when the driver intends to decelerate the vehicle.
However, the above-mentioned conventional braking apparatus is adapted to be applied to the regular engine in which a sufficient intake manifold negative pressure is generated by the throttle valve being closed when a vehicle is decelerated. Therefore, if the above-mentioned conventional braking apparatus is applied to the direct-injection engine, it is possible that a sufficient braking force cannot be generated due to insufficient negative pressure in the brake booster when the driver performs a brake operation.